1. Field of the Invention
The present invention relates to an ultrasonic observation apparatus having an ultrasonic endoscope capable of being inserted into a body of a patient and taking ultrasonic tomographic images for medical diagnoses.
2. Description of a Related Art
Recent years, an ultrasonic endoscope to be used by being inserted into a body cavity of a patient has been developed for medical diagnoses based on ultrasonic tomographic images around the body cavity of the patient. In an ultrasonic endoscope, a mechanical radial scan method has been generally adopted which performs the scan with viewing angle of 360° by mechanically rotating an array including plural ultrasonic transducers (ultrasonic vibrators) for transmitting and receiving ultrasonic waves.
Further, an electronic radial scan method has been proposed which performs the scan with viewing angle of 360° by electronic scan. For example, Takeda et al., “The Experience of Electric Radial Endoscopic Ultrasonography on Gastroenterology”, Jpn J Med Ultrasonics, Vol. 31, Supplement (2004), 77-C062 discloses results from studies on efficacy and problems of an electronic radial ultrasonic endoscope based on experiences in using the electric radial ultrasonic endoscope (ER-EUS) for various digestive system diseases.
Furthermore, in the radial scan method, acquisition of high quality ultrasonic images by simultaneously transmitting plural ultrasonic waves having frequencies different from one another in plural different directions has also been proposed.
As a related technology, Japanese Patent Application Publication JP-P2001-333902A discloses an ultrasonic diagnostic apparatus for obtaining plural pieces of echo data for observation by performing ultrasonic scan while varying an ultrasonic wave frequency. This ultrasonic diagnostic apparatus includes as main parts an ultrasonic observation unit, an image processing unit for performing various kinds of image processing, an ultrasonic probe having ultrasonic vibrators for transmitting and receiving ultrasonic waves, and a driving unit for driving the ultrasonic probe, and the ultrasonic vibrators are wideband vibrators such that driving frequencies can be changed (page 1). In the ultrasonic diagnostic apparatus, the ultrasonic observation unit includes a transmission and reception part for transmitting ultrasonic waves having different driving frequencies to the ultrasonic vibrators and receiving reflection waves of the ultrasonic waves, and a frame memory for receiving and storing the reflection waves as echo data. The selection among the various driving frequencies generated in the transmission and reception part is controlled by a system controller, and the selected driving frequency and echo data when employing the driving frequency are stored in association with each other.
Japanese Patent Application Publication JP-A-8-173420 discloses an ultrasonic image processing apparatus for synthesizing plural ultrasonic images having different frequency characteristics to make a smooth image and displaying the smooth image. This ultrasonic image processing apparatus includes ultrasonic transmitting and receiving means having different frequency characteristics from one another, control means for driving the plural ultrasonic transmitting and receiving means, and display means for weighting reception signals outputted from the plural ultrasonic transmitting and receiving means according to distances from positions within a screen corresponding to the ultrasonic transmitting and receiving means and synthesizing the reception signals to display the image in one screen (page 2).
Japanese Patent Application Publication JP-A-5-56980 discloses an ultrasonic diagnostic apparatus for easily performing frequency selection so as to deal with ultrasonic attenuation caused by higher frequency. This ultrasonic diagnostic apparatus includes an ultrasonic transducer shared among at least two center frequencies for obtaining reception signals by transmitting and receiving ultrasonic pulses having respective center frequencies, and means for independently collecting the reception signals for each center frequency by using the ultrasonic transducer (pages 1 and 2).
Japanese Patent Application Publication JP-P2001-170046A discloses a living tissue property diagnostic apparatus including signal analysis means for receiving an ultrasonic pulse reflected or transmitted within a living body and converting it into an electric signal, and diagnosing the living tissue property from an amount of characteristic of the electric signal in order to make correct diagnoses regardless of targets of measurement. In this living tissue property diagnostic apparatus, the signal analysis means has pulse width setting means for setting a pulse width of an electric signal, region extracting means for extracting plural signal regions which are different at least in part from one another, waveform characteristic value calculating means for calculating a predetermined waveform characteristic value in each of the extracted regions, difference computing means for computing the difference between the calculated waveform characteristic values, and corresponding time determining means for relating the result of the difference computation to the position of the living tissue that has generated the reception ultrasonic pulse by associating the result of the difference computation with the reception time of the ultrasonic pulse.
Thus, according to the mechanical or electronic radial scan method, there is an advantage that a wide viewing angle can be obtained. However, in comparison to a scan method of performing scanning with viewing angle of 90° by using a convex ultrasonic transducer array, for example, in the case where conditions of sound ray density, scan depth, and so on are set equal, there is a problem of reduction in frame rate because scan time (frame period) for obtaining an image for one frame becomes longer.
Further, as disclosed in JP-P2001-333902 and JP-A-8-173420, there is an advantage that the high resolving power (resolution) in the shallow part and penetration (focal depth) of ultrasonic waves to the deep part are consistent with each other by synthesizing plural ultrasonic images obtained by using ultrasonic waves having different frequencies from one another. However, in JP-P2001-333902 and JP-A-8-173420, only the mechanical radial scan method is used but it is not considered to use the electronic scan method.
On the other hand, JP-A-5-56980 suggests use of the electronic scan method when respectively transmitting plural ultrasonic waves having different frequencies from one another in plural different directions (page 3), but no specific aspect is revealed. In the case where the electronic scan method is adopted thereto, it is conceivable that the number of connecting lines to ultrasonic transducers becomes larger, but there is no disclosure about the measure against the problem. Also, there is no disclosure about combination of plural ultrasonic images obtained based on ultrasonic waves having different frequencies from one another to display the image, nor combination with an ultrasonic image generation method such as Doppler method other than B-mode image.
Further, JP-P2001-170046A does not specifically disclose an aspect of a probe to be used for extracting frequency components.